ANTHOZOA ALCYONARIA. 739 



whilst the small tubes lodge the siphonozooids. The calcite is laid down in 

 lamellae and it is coloured blue by an organic pigment. The surface of 

 the skeleton is covered superficially by a layer of cells or calycoblasts 

 from which the hard structures are derived l . 



Skeletal structures to be noted are the external and internal tabulae 

 of Tubiporidae, the septa and tabulae of Helioporidae. The external 

 tabulae ( platforms) of Tubipora are developed as rims close to the 

 oral extremities of the zooids. These rims either surround neighbouring 

 tubes or fuse with adjacent rims. They are at first soft and consist of 

 ectoderm with a core of mesoglaea, but as they increase in size endodermic 

 canals spring from the gastric cavities of the zooids and ramify in the 

 mesoglaea. Spicules are formed at the same time and gradually unite 

 into a firm skeleton. The internal tabulae are formed by a shrinkage, at 

 the level of the platforms, of the endoderm and the lamina of mesoglaea 

 lining the skeletal tube and the simultaneous formation of layers of 

 spicules on the surface of the mesoglaeal lamina. These tabulae assume 

 various shapes. The calcareous septa of Heliopora are ridges projecting 

 inwards at the mouths of the calycles, usually twelve in number, but some- 

 times more, up to sixteen. The internal calcareous tabulae or horizontal 

 floors are flat-bottomed cups added within the cavities of the calycles and 

 tubes. In both genera the internal tabulae limit the deep dying or dead 

 region of the colony from the superficial and growing region. 



As to the zooids themselves, when the mesoglaea of the colony is plentiful 

 their anterior or oral extremities are invaginable ; when it is scanty they 

 are usually only contractile. The tentacles are hollow and set one over 

 each of the eight perigastric chambers ; they are pinnate, and are some- 

 times invaginable as in Corallium and Heliopora, but are usually only con- 

 tracted when the fore-part of the zooid is invaginated. The oesophagus or 

 stomodaeum is of some length ; its walls are transversely folded in inva- 

 gination. The siphonoglyphe is absent in all the Axifera hitherto ex- 

 amined, probably also in the non-colonial genera. It is well developed 

 in the Alcyonidae, feebly however in the autozooids of Sarcophyton. It is 

 wanting in the autozooids of Pennatulidae^ of Heteroxenia, and Paragorgia, 

 but present in their siphonozooids. The mesenteries are thin. The re- 

 tractor muscles, which are usually well-developed, are borne upon the 

 homologous aspect of each mesentery, in such a way that the two me- 

 senteries which limit a chamber corresponding to one extremity of the 

 mouth have the muscles on the surfaces turned to each adjoining lateral 

 chamber, whilst the two limiting the chamber at the opposite extremity of 



1 The calycoblasts of Heliopora give origin, according to Professor Moseley, to a fibrilloid 

 organic substance which undergoes calcification, and is afterwards removed (?), seeing that little or no 

 organic matter is to be detected in the older parts of the skeleton. Judging from Prof. Moseley's 

 figures and description, the calycoblasts are of undoubted ectodermic origin. 



